Atomic Spectra: Study of Symmetrical Lines from n=1

[M. next]

When doing an experiment in order to observe the atomic spectra of some gas. We observe while peaking through the diffraction grating, symmetrical lines starting from n=1. Where does this symmetry arise from? Why do we see the same spectrum repeated on both, left side and right side. As if there was a mirror instead of the gas tube that reflected the first spectrum to the opposite side.

Thanks.

 

[mfb]

The setup is symmetric, there is no reason why the lines should be asymmetric.

 

[M. next]

Thank you for your reply.